1. Field of the Invention
The present invention relates to a zinc-air secondary battery.
2. Description of the Related Art
One of the promising candidates for an innovative battery is a metal-air battery. Since oxygen, which participates in cell reaction, is supplied from the air into the metal-air battery, the metal-air battery can make the best use of the space in a battery cell for packing a negative electrode active material, thus making it possible to achieve a high energy density in principle.
The majority of the metal-air batteries currently proposed is a lithium-air battery. However, the lithium-air battery has many technical problems, such as formation of undesired reaction products on an air electrode, incorporation of carbon dioxide, and short-circuiting between positive and negative electrodes due to formation of lithium dendrites (dendritic crystals).
On the other hand, a zinc-air battery, in which zinc is used as a negative electrode active material, has also been known from the past. In particular, a zinc-air primary battery has already been mass-produced and widely used as a power source for a hearing aid or the like. The zinc-air battery uses, as an electrolyte solution, an alkaline aqueous solution such as potassium hydroxide, and uses a separator (partition) for preventing short-circuiting between positive and negative electrodes. During discharging, O2 is reduced in the air electrode (positive electrode) to produce OH−, while zinc is oxidized in the negative electrode to produce ZnO, as shown by the following reaction formulae:Positive Electrode: O2+2H2O+4e−→4OH−Negative Electrode: 2Zn+4OH−→2ZnO+2H2O+4e−
Although some attempts to use this zinc-air battery as a secondary battery have ever been made, there is a problem that deposition of zinc metal occurs as dendrites by reduction of ZnO during charging, which penetrate the separator to come into contact with a positive electrode, thus causing short-circuiting. This problem has been a big hindrance to practical application of the zinc-air battery as a secondary battery.
Moreover, there was also a problem that carbon dioxide in the air permeates through the air electrode and dissolves in an electrolyte solution to produce alkaline carbonate, thus degrading a battery performance. To deal with this problem, there has been made an attempt to provide the inside of the air electrode with an anion exchange membrane, through which hydroxide ions generated in the air electrode permeate and by which cations, such as an alkaline metal ion (e.g., K+) and a negative electrode metal ion (e.g., Zn2+) in an alkaline electrolyte solution, are prevented from permeating the air electrode side, so as to suppress precipitation of carbonate (K2CO3) and metal oxide (ZnO), which are otherwise produced in the air electrode by a chemical reaction with carbon dioxide in the air (see e.g., Patent Document 1 (WO2009/104570)). However, the anion exchange membrane is made of resin, and thus has a problem that zinc dendrites, which are formed on the negative electrode during charging, penetrate the anion exchange membrane to come into contact with the air electrode, so that short-circuiting may occur between the positive and negative electrodes.
In order to suppress the formation of dendrites in a secondary battery, an attempt to allow an electrolyte solution to contain a dendrite formation inhibitor has also been proposed (see e.g., Patent Document 2 (JP2009-93983A)). However, such an attempt does not address the problem of carbon dioxide incorporation.
The zinc-air battery has less problems with reaction than the lithium-air battery. Thus, it is said that if the problems with short-circuiting between the positive and negative electrodes caused by zinc dendrites and carbon dioxide incorporation are solved, the zinc-air battery would be highly feasible as a high capacity secondary battery. Therefore, in the zinc-air secondary battery, a technique for preventing both of the short-circuiting caused by zinc dendrites and the carbon dioxide incorporation is highly desired.
In the meantime, as a solid electrolyte having hydroxide ion conductivity, layered double hydroxides (LDH) represented by the general formula: M2+1-xM3+x(OH)2An−x/n.mH2O (wherein M2+ is a bivalent cation, M3+ is a trivalent cation, and An− is an n-valent anion) have been recently known, and the use of a membrane of the layered double hydroxides as an alkaline electrolyte membrane of a direct alcohol fuel cell has been proposed (see e.g., Patent Document 3 (WO2010/109670)).